1. Field of the Invention
The present invention concerns a method of producing reduced iron agglomerates by reducing iron oxide agglomerates incorporated with carbonaceous material.
2. Description of the Background
The MIDREX method is well-known as a method of producing reduced iron. In the MIDREX method, a reducing gas produced from natural gas is fed through a tuyere into a shaft furnace and allowed to rise therein for reduction of iron ores or iron oxide pellets charged therein, to thereby produce reduced iron. However, since the method required a supply, as a fuel, of a large amount of high-cost natural gas, the location of a plant utilizing the MIDREX method is limited to a region producing natural gas.
In recent years, a certain type of method for producing reduced iron has become of interest, in which relatively inexpensive coal can be used instead of natural gas as a reducing agent. An example of a method of producing reduced iron is disclosed in U.S. Pat. No. 3,443,931, which is hereby fully incorporated by reference. In this prior art technique, a mixture of powdery of iron ore and carbonaceous material is pelletized and, then reduced in a high-temperature atmosphere in a rotary hearth furnace with a flat annular hearth to thereby produce reduced iron. This method has advantages in that coal can be used as a reducing agent, a powder of iron ore can be used directly, reduction can be performed at a high rate and the carbon content of products can be regulated. Since the pellet temperature at the end of reduction is in the order as high as 1300.degree. C., it is necessary to lower the temperature of the reduced pellet below 1000.degree. C. in the reducing furnace for easy handling in order to fabricate the reduced pellets into final products such as hot briquettes. For this purpose, U.S. patent described above proposes a method of disposing a cooling zone at the final end in the reducing furnace, and cooling by radiation the reduced pellets at high temperature to lower than 1000.degree. C. by using a water cooling panel or the like. In this method, a pellet incorporated with carbonaceous material is heated and iron oxide is reduced by CO gas liberated from the incorporated carbonaceous material. Therefore, reduction proceeds at the inside of the pellet and the iron oxide is metallized with no substantial effect of a combustion gas atmosphere.
However, referring to cooling of the pellet after reduction, the method of disposing the cooling zone at the end of the reducing furnace as described in this U.S. patent involves a problem that the effective hearth area for reduction is decreased by so much as the cooling panel is disposed in the cooling zone, to lower the productivity of the reducing furnace.
Further, the method also involves a problem that the surface layer of the pellet is not reduced if the combustion gas atmosphere is oxidative, so that the surface layer for 0.5 to 1.0 mm area is left as the iron oxide and the degree of metallization remains in the order of 80%. As a countermeasure, in the method described in the U.S. Pat. No. 3,443,931, inside of the furnace is divided into three heating stages, a heating temperature is defined for each of the heating stages and the volume percentage of burnable components (H.sub.2 +CO) in the atmosphere is controlled along with the progress of reduction, thereby promoting the reduction at the surface layer of the pellet and increasing the degree of metallization of reduced iron. However, the degree of metallization is not yet satisfactory.
It is considered that not only the volume percentage of the burnable component but also the ratio thereof with the oxidative gas, that is, degree of reduction are important in order to increase the degree of metallization, so that improvement for the degree of metallization can not be expected only by defining the volume percentage of the burnable components if the kind of the fuels differs. Further, the method of controlling the degree of reduction of the atmospheric gas involves a problem in that it requires change of the composition of the great amount of gas in the entire combustion chamber and, accordingly, gives a negative effect of lowering the combustion efficiency to require a pellet retention time of more than as three minutes in the atmosphere control zone.
Further, Japanese Patent Publication Sho 45-36092 which is hereby fully incorporated by reference discloses a method of producing metallized pellets by the steps of mixing powdery iron ore, powdery carbonaceous material such as powdery coal and powdery flux such as limestone, together with a predetermined flux so as to form an intimate powdery mixture, pelletizing them into individual spheres and drying the mixture in a sufficiently inert atmosphere on a moving grate type machine. The patent describes that the content of gaseous materials used for preventing re-oxidation of once reduced metallic iron has to be kept to less than a predetermined limit and, specifically, that a non-reactive gas with the oxygen content below 15% by weight is used.
However, when the temperature at the center of the pellet exceeds 1000.degree. C., solution loss reaction; C+CO.sub.2.fwdarw.2CO becomes vigorous and a great amount of CO rich gas evolves from the inside of the pellet, so that reduction proceeds with no substantial effect of the combustion gas atmosphere and iron oxide is metallized. However, since iron oxide is left as it is or metallic iron is re-oxidized by the combustion gas in the surface layer of the pellet for a 0.5-1.0 mm region at the final stage of the reduction, there is a problem that the degree of metallization remains only in the order of 80%.